The invention relates to a process for the preparation of a polyamide comprising at least a step in which a composition that comprises at least (a) a primary aminocarbonamide and (b) an aminocarboxylic acid and/or a lactam is polymerised.
Such a process is for example known from WO-A-9943734, which describes the preparation of a polyamide-6 from a composition containing 6-aminocapronitrile, 6-aminocapronamide, 6-aminocaproic acid, xcex5-caprolactam and water, in the presence of a heterogeneous metal-oxide catalyst at elevated pressure and temperature.
The disadvantage of the process according to WO-A-9943734 is that no high relative viscosities can be obtained, in particular a polyamide-6 with a xcex7rel of more than 2.2. In the context of this application, xcex7rel is understood to be the relative viscosity measured in a 1 wt. % solution of formic acid. Another disadvantage of the process according to WO-A-9943734 is the presence of an amount of aminocapronitrile in the composition. A large amount of water, at least 50 wt. %, is required to hydrolyse and subsequently polymerise this compound. A large amount of water is disadvantageous with respect to obtaining a high xcex7rel in a polymerisation based on a 1-step polycondensation process. Another disadvantage is that several steps (at least 3) are required to obtain the polyamide according to WO-A-9943734. Another disadvantage of the process according to WO-A-9943734 is the need to use a catalyst to obtain a polyamide-6 with a xcex7rel of at least 1.6 and at most 2.2. A polyamide-6 with such a low xcex7rel is of little economic interest; in particular, it is not suitable for the production of fibres, films and moulded parts. Another disadvantage of the process according to WO-A-9943734 is that the polymerisation has to be carried out at elevated pressure, for example at a pressure of 55.105 Pa.
The aim of the invention is a process for the preparation of a polyamide that does not show the disadvantages according to the state of the art. In particular the aim of the invention is to prepare a polyamide-6 with a xcex7rel of more than 2.2.
The inventors have now surprisingly found that such a polyamide can be prepared if the composition comprises at least
(a) 10-90 wt. % primary aminocarbonamide;
(b) 10-90 wt. % aminocarboxylic acid and/or lactam;
(c) 0-4 wt. % water;
the amounts being relative to the sum of the compounds (a+b+c).
Preferably the sum of the compounds (a+b+c) is at least 75 wt. % of the total composition, more preferably at least 85 wt. %, and most preferably at least 90 wt. %.
Surprisingly, it was also found that even a small amount of aminonitriles had a major negative influence on the xcex7rel of the polyamide prepared with the process according to the invention. Preferably the composition therefore contains less than 0.1 wt. % aminocarbonitriles, more preferably less than 0.05 wt. %, most preferably less than 0.01 wt. %, the amounts being relative to the total composition. Such an influence was not described in WO-A-9943734.
Preferably the composition according to the invention contains 20-80 wt. %, more preferably 30-70 wt. % of the primary aminocarbonamide.
Preferably the composition according to the invention contains 20-80 wt. %, more preferably 30-70 wt. %, of the aminocarboxylic acid and/or lactam.
Preferably the composition according to the invention contains less than 2 wt. %, more preferably less than 1 wt. % water.
The primary aminocarbonamide present in the composition which is polymerized in the process according to the invention is a compound according to formula (I) 
where m=3 to 12, preferably m=4 to 7. A particularly suitable compound is 6-aminocapronamide (m=5).
The aminocarboxylic acid is a compound according to formula (II) 
where m=3 to 12, preferably m=4 to 7. A particular suitable aminocarboxylic acid compound is 6-aminocaproic acid (m=5).
The lactam is a compound according to formula (III) 
where m=3 to 12, preferably m=4 to 7. A particular suitable lactam compound is xcex5-caprolactam (m=5).
The composition according to the invention may optionally also contain aminocarboxylic esters, secondary or tertiary aminocarbonamides and oligomers of aminocarboxylic acid compounds. Examples of suitable aminocarboxylic esters are aminocarboxylic esters of which the ester group contains a C1-14 alkyl group, for example a methyl, ethyl, n-propyl, i-propyl, n-butyl, sec-butyl and tert-butyl group. The aminocarboxylic ester is for example 6-aminocaproic methylester and 6-aminocaproic ethylester. The oligomer is for example the cyclic oligomer of xcex5-caprolactam. Preferably the composition contains at most 25 wt. %, relative to the total composition, more preferably at most 15 wt. %, most preferably at most 5 wt. %, of such compounds.
The process is eminently suitable for the preparation of polyamide-6 (polycapronamide) from a composition of 6-aminocapronamide, 6-aminocaproic acid and/or xcex5-caprolactam. Such a composition can be obtained by mixing the aforementioned individual compounds, which are commercially available. However, a particularly suitable embodiment of the invention is that in which the composition is obtained in the reductive amination of a 5-formylvalerate ester, for example 5-formylmethylvalerate, for example according to the following reaction: 
Examples of such a reaction are described in EP-A-729943, EP-A-729944, U.S. Pat. Nos. 4,766,237, 4,730,041, 4,731,445 and 5,068,398. Such a composition typically contains amounts of (a) 6-aminocapronamide and (b) 6-aminocaproic acid and/or xcex5-caprolactam in an a/b ratio=3/1 to 1/3, in particular 2/1 to 1/2. Such ratios are eminently suitable for the execution of the process according to the invention. The mixture, directly obtained via the reductive amination, also contains an amount of water and an alcohol, for example methanol, which can however easily be removed in their entirety or in part, for example through evaporation or distillation. Such a composition (from which the water and the alcohol have been almost completely removed) can then be directly polymerised to obtain polyamide-6. The invention hence also relates to a process for the preparation of polyamide-6 in which the composition is obtained in the reductive amination of a 5-formylvalerate ester performed in the presence of water, preferably 5-formylmethylvalerate.
The 5-formylvalerate ester can for example be obtained from butadiene via (a) a carbonylation and (b) a hydroformylation, for example according to the following reactions: 
Examples of reaction step (a) are described in WO-9933779 and WO-9838151. Examples of reaction step (b) are described in WO-9518089 and WO-9733854. The invention hence also relates to a process for the preparation of polyamide-6 in which the composition is obtained from butadiene through successively (a) a carbonylation, (b) a hydroformylation and (c) a reductive amination of the mixtures of intermediates successively obtained.
The polyamide-6 prepared with the aid of the process according to the invention has a xcex7rel of at least 2.2 and a low concentration of cyclic dimer of xcex5-caprolactam.
A particular advantage of the process according to the invention is that no catalyst need to be used, as a result of which the end product (the polyamide) contains no catalyst residues, or the preparation does not comprise a step in which a catalyst must be removed from or deactivated in the reaction mixture or polyamide. Another advantage is that the preparation can be carried out in polymerisation installations that are used on a large technical scale for the polymerisation of xcex5-caprolactam, for example as described in Kunststoff Handbuch 1, Becker/Braun, Hanser Verlag, 1990, in particular in installations in which the polymerisation is carried out in a VK column (VK=Vereinfacht Kontinuerlich). In this case the composition according to the invention is supplied to the top of a vertical tube reactor or a series of tube reactors at a temperature of about 240xc2x0 C. to 275xc2x0 C. and at a pressure of (0.5-100). 105 Pa, preferably (0.5-20). 105 Pa, more preferably (0.5-1.1). 105 Pa. Under these conditions the polymerisation is in a first part of the column initiated by means of ring opening of the lactam (if present) under the influence of the water present, after which the polycondensation and polyaddition take place in the next part of the column, with water and ammonia being discharged. Residence times are in the order of 10-20 hours to realise a sufficiently high degree of polymerisation. During the polymerisation an equilibrium is established between the polymer, the monomers and oligomers, as a result of which the polyamide leaving the polymerisation column contains in the order of 10 wt. % monomers and 2 wt. % oligomers, in particular the cyclic oligomer of caprolactam. The total polymerisation time can be shortened by a few hours if use is also made of a prepolymerisation reactor ahead of the VK column, in which a low-molecular polyamide is prepared under comparable VK conditions.
The polyamide leaving the VK column can subsequently be after-condensed in the manner known to one skilled in the art to raise the relative viscosity to for example 4.0, as a result of which the polyamide becomes suitable for for example film applications.
The reaction mixture may also contain the usual chain length regulators and branching agents known to one skilled in the art, plus the usual additives such as titanium dioxide, nucleating agents, stabilisers, antioxidants, clay particles, glass fibres, nano-material and mixtures thereof.
Suitable for use as chain length regulators are monofunctional carboxylic acids, for example aliphatic and aromatic carboxylic acids, in particular propionic acid, acetic acid, benzoic acid and monofunctional amines, in particular cyclohexylamine, tridecylamine, benzylamine, triacetone diamine and mixtures thereof, difunctional carboxylic acids, for example aliphatic and aromatic dicarboxylic acids, in particular adipic acid, azelaic acid, sebacic acid, dodecanoic diacid, terephthalic acid, isophthalic acid and difunctional amines, in particular hexane diamine, xylylene diamine and aminodicyclohexylmethane.
Suitable for use as branching agents are compounds containing three functional groups that can react with the end groups of the polyamide, in particular N,Nxe2x80x2,Nxe2x80x3-(tris-caproic acid) melamine and dendrimers based on polyamines, for example as described in DE-A-19654179.
The invention also relates to the polyamide that can be obtained with the process according to the invention. In addition to the acid and amine end groups, this polyamide also contains a number of amide end groups.
The invention also relates to the use of the polyamide for the production of fibres, injection or extruded moulded parts or films as well as to the fibres, injection or extruded moulded parts or films obtainable from the polyamide obtainable according to the invention process.
The invention will be further elucidated with reference to the following examples, without however being limited thereto.